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Timeline of plant evolution
Plant evolution is an aspect of the study of , predominantly involving evolution of plants suited to live on land, greening of various land masses by the filling of their with land plants, and diversification of groups of land plants. Earliest plants In the strictest sense, the name plant refers to those land plants that form the , comprising the bryophytes and vascular plants. However, the clade or green plants includes some other groups of photosynthetic eukaryotes, including . It is widely believed that land plants evolved from a group of s, most likely simple single-celled terrestrial algae similar to extant in plants evolved from an relationship between a , a photosynthesising and a non-photosynthetic organism, producing a lineage of photosynthesizing eukaryotic organisms in marine and freshwater environments. These earliest photosynthesizing single-celled autotrophs evolved into multicellular organisms such as the , a group of freshwater green algae. Fossil evidence of plants begins around 3000 Ma with indirect evidence of oxygen-producing photosynthesis in the geological record, in the form of chemical and isotopic signatures in rocks and fossil evidence of colonies of cyanobacteria, photosynthesizing organisms. Cyanobacteria use water as a , producing atmospheric oxygen as a byproduct, and they thereby profoundly changed the early atmosphere of the earth to one in which modern aerobic organisms eventually evolved. This oxygen liberated by cyanobacteria then dissolved in the oceans, the iron precipitated out of the sea water, and fell to the ocean floor to form sedimentary layers of oxidized iron called s (BIFs). These BIFs are part of the geological record of evidence for the evolutionary history of plants by identifying when photosynthesis originated. This also provides deep time constraints upon when enough oxygen could have been available in the atmosphere to produce the blocking ozone layer. The oxygen concentration in the ancient atmosphere subsequently rose, acting as a poison for organisms, and resulting in a highly oxidizing atmosphere, and opening up niches on land for occupation by aerobic organisms. Fossil evidence for cyanobacteria also comes from the presence of s in the fossil record deep into the . Stromatolites are layered structures formed by the trapping, binding, and cementation of sedimentary grains by microbial s, such as those produced by cyanobacteria. The direct evidence for cyanobacteria is less certain than the evidence for their presence as primary producers of atmospheric oxygen. Modern stromatolites containing cyanobacteria on the west coast of Australia and other areas in saline lagoons and in freshwater. Paleozoic flora flora Early plants were small, unicellular or filamentous, with simple branching. The identification of plant fossils in Cambrian strata is an uncertain area in the evolutionary history of plants because of the small and soft-bodied nature of these plants. It is also difficult in a fossil of this age to distinguish among various similar appearing groups with simple branching patterns, and not all of these groups are plants. One exception to the uncertainty of fossils from this age is the group of calcareous green algae, found in the fossil record since the middle Cambrian. These algae do not belong to the lineage that is ancestral to the land plants. Other major groups of green algae had been established by this time, but there were no with vascular tissues until the mid- . flora The evidence of plant evolution changes dramatically in the Ordovician with the first extensive appearance of spores in the fossil record (Cambrian spores have been found, also). The first terrestrial s were probably in the form of tiny plants resembling s when, around the Middle Ordovician, evidence for the beginning of the terrestrialization of the land is found in the form of tetrads of spores with resistant polymers in their outer walls. These early plants did not have conducting tissues, severely limiting their size. They were, in effect, tied to wet terrestrial environments by their inability to conduct water, like extant , , and , although they reproduced with , important dispersal units that have hard protective outer coatings, allowing for their preservation in the fossil record, in addition to protecting the future offspring against the desiccating environment of life on land. With spores, plants on land could have sent out large numbers of spores that could grow into an adult plant when sufficient environmental moisture was present. flora The first fossil records of s, that is, land plants with s, appeared in the . The earliest known representatives of this group (mostly from the northern hemisphere) are placed in the genus . They had very simple branching patterns, with the branches terminated by flattened sporangia. By the end of the Silurian much more complex vascular plants, the , had diversified and primitive s, such as (originally discovered in Silurian deposits in Victoria, Australia), had become widespread. flora By the Devonian Period, the colonization of the land by plants was well underway. The l and algal mats were joined early in the period by primitive s that created the first recognizable s and harbored some arthropods like s, s and s. Early Devonian plants did not have roots or leaves like the plants most common today, and many had no vascular tissue at all. They probably relied on l symbioses with fungi to provide them with water and mineral nutrients such as . They probably spread by a combination of forming clonal colonies, and sexual reproduction via spores and did not grow much more than a few centimeters tall. By the Late Devonian, forests of large, primitive plants existed: s, , s, and s had . Most of these plants have true roots and leaves, and many were quite tall. The tree-like , ancestral to the gymnosperms, and the giant trees had true . These are the oldest known trees of the world's first forests. was the fruiting body of an enormous fungus that stood more than 8 meters tall. By the end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion". The primitive arthropods co-evolved with this diversified terrestrial vegetation structure. The evolving co-dependence of insects and seed-plants that characterizes a recognizably modern world had its genesis in the late Devonian. The development of soils and plant root systems probably led to changes in the speed and pattern of and sediment deposition. The 'greening' of the continents acted as a , and concentrations of this may have dropped. This may have cooled the climate and led to a massive . see . Also in the Devonian, both s and arthropods were solidly established on the land. flora of northeastern .}} from the of .}} land plants were very similar to those of the preceding Latest Devonian, but new groups also appeared at this time. The main Early Carboniferous plants were the (Horse-tails), (scrambling plants), (Club mosses), (arborescent clubmosses or scale trees), (Ferns), (previously included in the " ", an artificial assemblage of a number of early groups) and the . These continued to dominate throughout the period, but during , several other groups, (cycads), the (another group of "seed ferns"), and the (related to and sometimes included under the ), appeared. The Carboniferous lycophytes of the order Lepidodendrales, which were cousins (but not ancestors) of the tiny club-mosses of today, were huge trees with trunks 30 meters high and up to 1.5 meters in diameter. These included (with its fruit cone called ), , and . The roots of several of these forms are known as . The fronds of some Carboniferous ferns are almost identical with those of living species. Probably many species were epiphytic. Fossil ferns include and the tree ferns and . Seed ferns or Pteridospermatophyta include , , , and . The Equisetales included the common giant form , with a trunk diameter of 30 to 60 cm and a height of up to 20 meters. was a slender climbing plant with whorls of leaves, which was probably related both to the calamites and the modern horsetails. , a tall plant (6 to over 30 meters) with strap-like leaves, was related to the cycads and conifers; the -like inflorescence, which bore yew-like berries, is called . These plants were thought to live in swamps and mangroves. True coniferous trees ( , of the order Voltziales) appear later in the Carboniferous, and preferred higher drier ground. flora The Permian began with the Carboniferous flora still flourishing. About the middle of the Permian there was a major transition in vegetation. The swamp-loving lycopod trees of the Carboniferous, such as and , were replaced by the more advanced conifers, which were better adapted to the changing climatic conditions. Lycopods and swamp forests still dominated the continent because it was an isolated continent and it sat near or at the equator. The Permian saw the radiation of many important conifer groups, including the ancestors of many present-day families. The s and cycads also appeared during this period. Rich forests were present in many areas, with a diverse mix of plant groups. The s thrived during this time; some of these may have been part of the ancestral lineage, though flowers evolved only considerably later. flora flora On land, the holdover plants included the s, the dominant s, (represented in modern times by ) and s. The s, or seed plants came to dominate the terrestrial flora: in the northern hemisphere, s flourished. (a ) was the dominant southern hemisphere tree during the Early Triassic period. flora The arid, continental conditions characteristic of the Triassic steadily eased during the Jurassic period, especially at higher latitudes; the warm, humid climate allowed lush jungles to cover much of the landscape. dominated the flora, as during the Triassic; they were the most diverse group and constituted the majority of large trees. Extant conifer families that flourished during the Jurassic included the , , , , and . The extinct Mesozoic conifer family dominated low latitude vegetation, as did the shrubby . s were also common, as were s and in the forest. Smaller s were probably the dominant undergrowth. were another group of important plants during this time and are thought to have been shrub to small-tree sized. Ginkgo-like plants were particularly common in the mid- to high northern latitudes. In the Southern Hemisphere, were especially successful, while Ginkgos and were rare. flora , the earliest known carnivorous plant}} Flowering plants, also known as , spread during this period, although they did not become predominant until near the end of the period ( ). Their evolution was aided by the appearance of s; in fact angiosperms and insects are a good example of . The first representatives of many modern trees, including s, and s, appeared in the Cretaceous. At the same time, some earlier Mesozoic s, like s continued to thrive, although other taxa like died out before the end of the period. Cenozoic flora The Cenozoic is just as much the age of savannas, or the age of co-dependent flowering plants and insects. At 35 Ma, evolved from among the angiosperms. About ten thousand years ago, humans in the of the Middle East develop agriculture. Plant domestication begins with cultivation of founder crops. This process of food production, coupled later with the domestication of animals caused a massive increase in human population that has continued to the present. In Jericho (modern Israel), there is a settlement with about 19,000 people. At the same time, Sahara is green with rivers, lakes, cattle, crocodiles and monsoons. At 8 ka, Common (Bread) wheat ( ) originates in southwest Asia due to hybridisation of emmer wheat with a goat-grass, . At 6.5 ka, two rice species are domesticated: Asian rice, , and African rice . References Category:Timeline